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General Information
Model 8902A
CAUTION
The Power Sensor is irnproftwed against and is easily damagcd by sudden,
large overloads. Refer to Table I - I under R F Power, Sirpplemcntal Charac-
teristics, RF Power Ranges
of
H P 8902A Measuring Receiver with HP I 1 722A
Sensor Moditkc, for injormation
on
maximirm operating levels.
The broadband, low-noise R F Amplifier improves the sensitivity of the Tuned R F Level measure-
ment. The amplifier is normally bypassed in the other measurement modes.
CAUTION
A n RI: defecfor in the RF AmpliJim (nof shown in Figirrc I-2) caiws thc R F
AmpliJier
to
be bypassed when
an
overload occirrs. However, becairse
of
the
relatively long switching timc (approximately 30 mJ), the
R 1 :
A mplifcr can bc
damaged by
a
large overload. Refir to T a b k
1-1,
itndcr R F Inpiit, Optrating
Level, .for injormafion
on
maximitm opcraf ing Icvcl.~.
When the R F Peak Detector senses that the input signal level exceeds 1 W, it opens the Overpower
Relay. This is done without intervention of the Controller. The output from the R F Peak Detector,
read by the voltmeter, is used to set the Input Attenuator to optimize the level applied to the Input
Mixer.
The Input Mixer converts the input signal to the intermediate frequency (IF). For frequencies greater
than 10 MHz, the IF is 1.5 MHz with the Local Oscillator (LO) tuned 1.5 MHz above the input
frequency; an IF of 455 kHz can be manually selected for this frequency range. The 455 kHz IF is
selected automatically for input signals between 2.5 MHz and 10 MHz and for the Tuned R F Level
measurement at all frequencies. Below 2.5 MHz, the input passes directly through the Input Mixer
without down-conversion. (The Tuned RF Level measurement is invalid below 2.5 MHz.)
NOTE
For fhc inpitf signal
to
pass fhroirgh thc Input Mixer withoitt down-conversion,
the LO mirst sfill hc prtwnt to fitrn thc mixcr diodes
on.
A
n
LO j-cqiicncy
of'
101.5 MHz is arbitrarily iiscd. Thus fhc insfrirmenf will rcspond
f o
inpitt
jrqrrencies
of'
I00
or
103
M H z
as
well
as
jrqiicncics het ween 150 kHz and
2.5 MHz.
The instrument can be manually tuned to a desired signal even in the presence of larger signals,
although filtering may be necessary since low-frequency signals pass directly into the IF. The RF
High-Pass Filter can be inserted (via a Special Function) in the R F path for this purpose.
To measure the input frequency, the Counter measures the frequency of the LO and the frequency
of the IF from the output of the IF Amplifier and Filter. The Controller computes and displays
the difference between the two frequencies. For input frequencies below 2.5 MHz, only the IF is
counted, which equals the input frequency.
LO Circuitry
The LO drives the high-level port of the Input Mixer and is one of several inputs to the Counter.
The LO has four main modes of operation:
0
tuning to the frequency required to down-convert a signal whose frequency is entered from
the keyboard (manual tune mode),
0
automatically searching for an input signal, then tuning the LO to the frequency required to
down-convert the signal (automatic tune mode),
1-10
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